In recent years, attention has been given to vehicles such as a electric vehicle, a hybrid vehicle, a fuel-cell electric vehicle that use an electric motor as a drive power source in view of environmental issues. This kind of vehicle employs a rechargeable and dischargeable battery unit for supplying an electric power to an electric motor and for converting a kinetic energy into an electric energy for storage during a regenerative operation.
For improving drive performance such as acceleration performance and a continuous travel distance of the vehicle using the electric motor as the drive power source, it is desired to increase a charge/discharge capacity of the battery unit. A structure employing a plurality of battery units has been proposed for increasing the charge/discharge capacity of the battery units.
For example, U.S. Pat. No. 6,608,396 has disclosed an electrical motor power management system providing a desired high DC voltage level to a high voltage vehicle traction system. This electrical motor power management system includes a plurality of power stages each including a battery and a boost-buck DC-DC converter. These power stages are connected in parallel, and provide a DC power to at least one inverter. Also, this system includes a controller controlling the plurality of power stages to charge and discharge uniformly the batteries of the plurality of power stages so that the plurality of power stages keep the battery voltage for at least one inverter.
Meanwhile, a battery unit including a chemical cell stores an electric energy by using an electrochemical energy. Therefore, charge/discharge characteristics thereof are liable to be affected by a temperature. In general, charge/discharge performance of the chemical cell lowers with lowering in temperature, but a higher temperature may promote deterioration of the chemical cell. Therefore, the temperature of the chemical cell mounted on the vehicle is managed to stay within a predetermined temperature range.
A method using an entropy change caused by charging/discharging has been known as one of methods for implementing the temperature management of such chemical cells. More specifically, an entropy changes in the chemical cell in accordance with changes in a state of charge caused by charging/discharging, and the changes in entropy cause a heat generating reaction or a heat absorbing reaction. In particular, a quantity of the reaction heat is relatively large in a lithium ion battery and the like. The reaction (i.e., the heat generating reaction or the heat absorbing reaction) that occurs actually depends on the state of charge and a passed current. Therefore, the temperature of the battery unit can be managed by appropriately determining the battery current according to the state of charge.
For example, Japanese Patent Laying-Open No. 09-019074 has disclosed a charge control system that can keep a battery temperature at an appropriate value during charging. This charge control system includes a charger charging a battery of which chemical reaction caused during charging is a heat absorbing reaction, and also includes control means for controlling a charge current of the charger depending on a state of charge of the battery. The control means causes absorption or generation of heat in the battery based on a state of discharge, a battery temperature and charging conditions, and thereby controls the charge current to keep the battery temperature in the predetermined temperature range.
In the charge control system disclosed in the above Japanese Patent Laying-Open No. 09-019074, the temperature management of the battery unit can be performed only during the charging, as is apparent even from the title. Therefore, sufficient temperature management cannot be performed in a vehicle (e.g., a hybrid vehicle) that frequently switches a state between charging and discharging depending on drive situations. Further, during the execution of the temperature management in the battery unit and thus during the charging, it is impossible to respond sufficiently to a power request made by a load (e.g., motor), resulting in a problem that drive performance of the vehicle is restricted.